August herman pfund



A. H, PFUND.

COLORIMETRY 0F PAINTS AND PIGMENTS.

APPLICATION FILED JAN-14.1920.

1,350,386, PatentedAug.24,1920

Mo luv/1 UNITED STATE PATENT OFFICE.

AUGUST HERMAN PFUND, 0F BALTIMORE, MARYLAND.

COLORIMETRY OF PAINTS Mill) I-IGMENTS.

T 0 all whom it may concern Be it known that 1, AUGUST HERMAN Prom), a citizen of the United States, residing at Baltimore, State of Maryland, have invented certain new and useful Improvements in Colorimetry of Paints and Pig If, from an assortment of'papers, we,

should pick out the sheets which appeal to us as being white, we would be satisfied with our choice until we laid the sheets side by side. By comparison some would be yellowish, others bluish, etc., and it would be difficult to select the one sheet that was truly the White. Problems o f this character are con stantly arising in the paper, paint and tex tile industries. Up to the present time it has been sought to regulate the uniformity, (with respect to color) of a particular prodnot-,by establishig a series of standard samples defining the limits of tolerance. Leaving out'of consideration the question of reproduceability, the principal objection to be urged against this modeof procedure is that the standards themselves are subject to change with time. This difficulty has long been recognized and numerous attempts have. been'made to apply the methods of monochromatic and trichromatic colorimetry. However, these attempts have been unsuccessful, because of the fact that the dc parture from white is usually so slight that accurate measurements of this departure are practically out of the question.

In the commercial manufacture of zinc oxid, lithopone and other white pigments, it is aimed to produce a pigment that is white. This ideal is not always realized and it-fre- I quently happens that these supposed white pigments have a bluish, yellowish or pinkish tint; In order to safeguard the uni formity of roduct, certain standards of well defined pin ish, yellowish or bluish tint (the so-called tint limits) are compared with a given sample. If atint of this sample is I fainter than that of the standard tint limits,

Specification of Letters Patent.

Patented Aug. 24, 1920.

Application filed January 14, 1920. Serial Not 351,423

the product is considered suitable for use,

otherwise it is discarded. The comparison is carried out by preparing pastes of the pigments rubbed down in linseed oil. The pastes are spread side by side on a pallet and are then compared. Where the tint differences are large, this method. is entirely serviceable, but where the tints of-the stand ard and sample are nearly the same, it is often 'difiicult to tell whether a pigment belongs in the blue, yellow or pink category. Under different conditions of illumination ones judgment of the tint is subject to variations and even under the same conditions of illumination, different observers are often of different opinions.

The method and apparatus of the present invention is peculiarly adapted for determining the variations from white of any particular paint, pigment or the like. Nhite, as a color, is difiicult to define, and its defini tion is moreor less arbitrary. subjectively, white is the color in comparison with which any other color appears tinted. Objectively,

the light of the midday sun, (blue sky light excluded) reflected from a non-selective surface, is white. White can be defined in physical unitsonly through its spectral energy curve. In speaking of a surface of yellow or blue,. it istacitly assumed that white light is incident on the surface. If, therefore, a body reflects selectively, it is to be called colored, whereas if it reflects non-selectively, it is to .;be called white. According to this nomenclature it is entirely proper to speak of one surface as being brighter than another ordeparting more from white. The term whiter than may not be used because the two concepts (1) increase in brightness, and (2) decrease inl departure from white (saturation), are both involved. j

In accordance with the present invervtion, the characteristic tint of the particular paint, pigment or the like, is much access tuated for the purposes of comparing this tint with a predetermined standard. Thus, a

in the case of supposed white paints or pigments, the departure of'the supposed white paint or pigments from true, white is very greatly accentuated, wherebylits true character can be established beyond doubt; The manner in which a. characteristic tint; is accentuated in accordance with the principles of the present invention, may be illustrated by considering the case of light reflected from a surface of polished copper. This metal owes its ruddy hue to its ability to reflect red light more strongly than any other color. Let us assume, for the sake of illustration, .that a Polished surface of copper reflects 100% o the red constituent in da light and;.50% of the blue constituent. f thelight reflected from a first copper surface or mirror be in turn reflected from a second and still a third copper surface or mirror, we shall have a state of affairs shown in the followingtable:

It is thus evident that after the third re- Red. Blue.

of daylight .f. 100 100 power 01 copper 1 0.5 alter reflection from first mirror. 100 50 after reflection from second mirror 100 after reflection from third mirror 100 12. 5

flection the red light'is relatively very much more intense than is the blue. In consequence of this phenomenon the copper appears of a deep ruby red color. Thus, by a method of multiple reflections the tint or color has been accentuated.

The principle of multiple reflections lends 'itsel'f admirably to the determination of smal departures from true white of various so-called white paints and pigments. In accordance with the present invention, the principle of multiple reflections is employed in order to accentuate this departure from contain an excess of ,green.

white. If white light is allowed to fall on a surface that is slightly greenish, it is obvione that the diffusely reflected light will If, now, this light is allowed to fall on a second surface, 1

entical with the first, the light reflected I will be, relatively, still richer in' green. Al- I lowing similarly a third-and fourth reflection to take place, it is possible to accentuate' salts, etc.) by the method and apparatus of the invention, not a single one .has been found that'does. not depart noticeably from white. I

The novel features of the invention which I believe to be patentable are definitely pointediout in the appended claims. These features, together with the construction and mode of operation of an apparatus embody- ,in the same, will be understood from the 0 lowing description taken in conjunction 1 with the accompanying drawings, in which:

Figure 1 isan elevation, partlyin seca sectioiial detail of this appa- 'matically represented a cylindrical tube 10 having its inside surface, coated with the material to be tested. An incident ray of light a is directed upon the coated inside surface ofthis tube and after three reflections the multiply reflected light I) is obtained. In Fig. 3 there is diagrammatically represented an annular disk 11 appropriately spaced from a somewhat larger circular disk 12. Incident rays of light a, directed upon the outer surface of the disk 12,

' emerge, after three reflections, through the central opening of the annular disk 11 as a reflected ray 7). It will, of course, be understood that the disks 11 and 12 have their adjacent orLfacing surfaces coated with the material to be tested. In Fig. 5 a disk 13,

having both. of its faces coated with the material to be tested, is positioned within a hollow sphere 14: having its inner surface coated with the material to be tested. With this arrangement, an incident ray of light a, entering the sphere through an opening at the top, is reflected four times before emerging as a reflected ray 6 through, an appropriate opening at the bottom of the sphere. The condition sought in all cases is that no light leaving the surface after the first reflection shall illuminate directly that portion from which the light finallyv emerges. Thus, for example, it will be observed that the arrangement shown in Fig.

6 is faulty, because the incident ray of light striking the inside coatedsurface of a concave member 15 undergoes three reflections throu'fgh',the instrumentality of the disk 16 and only two reflections through the member 15 when emergingas the reflected ray 7). Thus, the reflected ray is composed of rays which have undergone both two and three reflections and unreliable results may thereby be secured. By reversing the member 15 so that it occu ies the position represented by 15' in'Fig. et the drawings, a ,satisfactory arrangement for. three reflections is secured. The reflecting surfaces in these cases are not polished and the reflection from them is therefore diffused. The arrows are used to indicate merely directions taken by some of the diffusely reflected light.

'While it is true that the accentuation of atint increases exponentially with the number of reflections, a limit is set because of the resultant decrease in intensity. Furtliermore, in practice, preference must be given that design whichlends itself most readily to application of the surface to be.

studied. Taking all of these various eleti merits under consideration, the design, diagrammatically represented in Fig. 3, has

been chosen by me as the preferred arrange ment in the apparatus herein particularly described and illustrated. For the purposes of simplicity, I have termed this apparatus a colorimeter. v

The degree of accentuation may be calculated with readiness by studying the change in brightness (co-efi'icient of diffuse reflection) resultant upon the successive introduction of two different surfaces into the .color'nneter. Let

I :intensity of incident light. I, :intensity of light after a reflections 2,0. g from material 1.

L -fzintensity of light after n'reflections from material 2. R,:coeflicient of diffuse reflection of material 1 (known). p 535 li zcoeficient of difi'use reflection of material 2 (known). 'nznumber of reflections.

Y :instrumental constant.

'lh'en: V use:

.5 LKR.

or p lmfi li at 2 2 Whence log 77,: I log-- 2 I For a substance of unknown brigh -ness R p I. E to R3=RI Now R and R may be determined-by means of a Nutting (absolute) reflectometer 5 or by means of the usual methods of photometry, in which caseuse is made of a surface smoked with magnesium oXid,-wh.ose reflecting power has been determined. (87%) and whichserves as a standard of bright-- 5 ness. The intensities I, and I, are determined with the new colorimeter. T he value of 11 is determined by. the relative sizes of the colorimeter plates and of their-separation. For separations of the order of the my diameter of the upper disk n:3 or slightly greater; as the separation decreases, or becoines larger. For the actual tests here recorded, the separation was such as to yield the value 71:3.6 while in a series of tests at; later on with a smaller separation 12:42.

ately leveled by' screws 21.

shaped arm 23 by means of a rod 26 and If we were dealing with specular reflection,- n would necessarily have to be a -wliole.number. However, we are dealing with diffuse reflection and, while a must equal at least 3, it may be considerably larger due to the numerous reflections which take place. Since the-human eye cannot detect differences in intensity less than 1 to 2% (Fech- .ners law) it is clear that the method of multiple reflections makes possible a discrimination between two surfaces whose brightness difierence is, roughly, one-fourth of theabove.

Considerations of a similar nature apply to the determinationof-the departure from white or saturation, 2'. e., if green light be in excess by 1% after a first reflection, the

excess will-be approximately 4% after multiple reflections have taken place. Considering the truly marvelous sensibility of the 35 eye in distinguishing color differences between two contiguoussurfaces of large area, it would be rash to state that the new colorimeter is able to determine color differences between two surfaces which, under normal conditions,appear to be of the same brightness and tint. This muchmay be said, however: If the colorimetric analysis of the two surfaces is the same, the unaided eye may be able to distinguish a difference Without being able to define the character'of that difference.

Referring now to Fig. 1 of theacconr panying drawings, 1 will describe the con struction and mode of operation of an ap .paratus embodying the principles hereinbefore explained. The apparatus is mounted on a tripod base 20 adapted to be appropri- A U-shaped support or bracket 22 restsv on the to of the base 20. An L-shaped arm 23 is adgustably' secured to one of the arms of the bracket 22 by means of winged nuts 24. A cylindrical shield 25 is adjustably secured to the L- nut 27. Within the shield '25 is mounted a source of light 28. I have used a 100 watt Mazda, lamp with satisfactory results .as the source of light 28.

The light, diffusely reflected from the lamp 28, illuminates the surface of a circular disk 12, which in turn illuminates the central portion of an annular disk 11'. The disks 1L and 12 arearranged in substantially the same manner as the disks 1'1 and 1:20

12 of'Fig. 3, and, as in the case of Figfiii, the adjacent or facing surfaces of these disks are coated with the material to be tested.

A photometer cube 29 is positioned above I the central o'pening in the annular disk 11' and is appropriately inclosed within a housing 80. Cylindrical tubes 31 and 32 enter the housing 3O fIOHI opposite directions.- An enlarged cylindrical chamber 33 is atdotted line 0.

reflected light rays 6 and 0, While the 1I1(l the table is secured tached to the outer end of the tube 31. A disk of clear optical glass 34, roughly ground on both sides, is pivotally mounted Within the chamber 33. This glass disk 34 may be turned about its pivot by means of an operating handle 35. A pointer 36 is secured to the disk 34 and moves therewith,- and the position of this pointer, with re spect to a scale 37, indicates the inclination of the disk 34.

The tube 32 has an appropriate lens 38 mounted therein and at its outer end is provided with a viewing or eye piece 39. Intermediate the lens 38 and eye piece 39 the tube 32 is slotted, so as to receive apiece of glass 40, as will be more clearly understood by reference to Fig. 2 of the drawings.

The disk 12 is mounted on a circular table 41 attached to the upper end of a rod 42. The height of the table 41 is adjustable at any desired he'i ht by means of the screw 43. T he shield has a lateral opening 44 through which light from the .lamp 28 passes to the surface of the disk an opening 45 in the top of the chamber 33. The interior of the shield 25, as well as the interior of the other inclosed chambers and tubes of the apparatus, is covered with black velvet or equivalent material for absorbing light and preventing objectionable reflections.

The light from the lamp 28, after three reflections between adjacent faces of the disks 11.and 12', passes upward through the central opening in the annular disk 11 and is reflected horizontally (toward the right, as viewed in Fig. 1 of the drawings) by means of the photometer cube 29.. This reflected ray of light is diagrammatically represented in Fig. 1 of the drawings by the Another beam of light from the lamp 28 passes through the ports or openings 44 and 45 and is reflected from the disk 34 and passes through the tubes 31 and 32, as diagrammatically represented by the dotted line 4!. It is to be observed'that the photometer cube 29 is arranged to reflect all rays of light striking the surface 29 and to transmit all other'rays of light. Thus, that portion. of. the reflected rays 1) (of the incident rays a) which strike the reflecting element29' of the photometer cube, will be re- 'flected through the tube 32.

By this arrangement the lower half of the field of View through the tub'e 32 contains the multiply upper half of the field of viewof the tube 32 contains the rays 03, which are non-selectively reflected'from the tical glass disk 34.

The intensity of the light rays d, re-

flected from the disk 34, may be varied by rotating the disk 34 about itspivotal .axis.

34, through monochromatic light ground surface of the op- Numerical values of intensities are obtained as a result of separate calibration involving the removal of the disk 12and the admisto unduly complicate the drawing, this cali bratin device is not illustrated. The intensity o the light reflected from the disk 34 may be varied in the ratio of 1 :15. Whenever it becomes necessary to extend this Y range in either direction, a wire gauze coated with soot may be introduced either at R or I Q. Since the percentage transmission of the gauze may be regulated by choosing the appropriate ratio of width of mesh to diameter of wire, it is possible to take care of-all possible cases.

The photometer cube is viewed through va simple eye piece yielding a linear magnification of 2.5. penings are provided in the eye piece tube for the. insertion of a piece of daylight glass which transforms the yellowish li ht from the tungsten lamp into White lig 1t.

In order to produce a color match, a beam of monochromatic light, emanating from the secondary slit S, of a monochromatic illuminator, is projected on the disk 34 and passes through it, whereby monochromatic light is superimposed on the white light from the lamp 28. The intensity of the former is varied by means of a rheostat in series with the lamp illuminating the slit of the monochromatic illuminator. Color matching'is not a simple operation, since three variables (1) intensity of white light, (2) intensity of colored length of colhred light, have to be controlled. Proficiency is re'achqd only after Ion experience.

ollowing the usual methods of monochromatic colorimetry, t e color characteristics of any surface; (sir gle reflection) are defined in terms of:

light, and wave 1. Brightness (B), coeflicient of diffuse reflections.

2. Saturation (S), ratio of intensity of to total light required to produce.a color match.

3. Dominant hue (l), monochromatic light.

Since the necessary mode of procedure for obtaining the brightness (B) has already been discussed, it is only necessary to take up the matter of saturationKS).

Let

I =intensity balance light from surface under investigation (simple reflection).

I =intensity of white light necessary for color match.

of white light required to i wave-length of tions, is definitely yellowish.

l -Intensity of colored light Then I 0 s 1 IA whence I i I}\ I I1 Then I I I, S '1, I; T,

Accordihg to this procedure, I, need not be measured, hence the introduction of a flicker-photometer becomes unnecessary.

The value of l is obtained from the calibration curve of the monochromatic illuminator. If, instead of a single reflection, at reflections are realized, then the saturation is accentuated and it is necessary to extract the nth root of the apparent saturation so as to reduce the measurement to the conditions under which the surface is normally viewed, 2'. e., the first reflection.

Whenever the departure from white is marked, 6. 0., when the true saturation is 5% or more, readings are easily reproducible. Intensity measurements rarely differ by more than 1% and the dominant hue can be fixed to within 1 to 1'.5p.u. when saturation values are small (less than 0.7%) measurements become increasingly v difiicult particularly in fixation of the wave length of the dominant hue.

The following table contains data on the.

color characteristics of a number of nearlywhite substances:

- Domi- Substance. gg fi g nant hue Remarks.

7 in pa.

' Per cent. Per cent. M a g n c s i um 85 0. 2 486 Blue-fumes from oxid. burning magneslum. M a g n e s i um 87 0. 75 578 Ye1lowscrapings carbonate. from commercial black-flattened with glass. Paper 67 0. 70 489 Bluc-singlo sheet l1ncnpaper (thin). Paper 74 3. 0 581 Y ell 0 w sing le sheet linen paper (heavy). Whitc lead. 69 3. 8 580 Yellowr u b b e d down with pale linseed oilpaste rm. Zinc oxid 76 1.0 562 Grecn-rubbed (pure). ,down with pale linseed oil-paste form. Zinc oxid 67 7.1 583 Yellow-rubbed (leaded). down w th pale linseed oil-paste form.

All of these substances depart markedly from white. Even magnesium carbonate, which has been used frequently as anonselective reflector in colorimetric investiga- Following the well known practice, the procedure may be simplified by eliminating However,

measurements of saturation and dominant hue and instead, determining the bright ness for blue, green and red light. Color screens of dominant hue; 4401141; (blue), 550 (green) and 625cc (red) may, for 70 example, be placed in succession before the eye piece and the brightness for these three colors determined. While this procedure lacks the scientific rigor of the formerg it possesses marked advantages. First of all, no standard white light is required, second, no color matching is necessary, and third, it serves admirably in establishing. limits of tolerance, that is, selectively, for substances which are already nearly white.

The advantages derived from -multiple reflections need not be sacrificed when markedly colored surfaces are to be studied. The only difficulty in such' circumstances is to get enough light for accurate measure-' ment. The theoretical and scientific principles of operation of the apparatus of the invention will be clear to those skilled in the art from the fgregoing discussions In general, it will .be observed that the apparatus comprises I illustrated, the multiply-reflected beam of light is juxtaposed in the field of View with a standard beam of light derived from the common light source (28). This standard beam of light. is non-selectively reflected from the coarse-groundsurface of the disk or plate' 34 of optical glass, and the intensity of this standard beam of light can be varied by adjusting the inclination of the disk 34. In the field of view, as observed through the eye piece 39, the two juxtaposed beams of light appear as sernicircular segments, the line of demarcatmn between these segments being susceptible of variation by adjusting the inclination of ,the disk 34. In the use of the apparatus,

the disk 34 is adjusted until the two uxtaflections from surfaces of the tint t be accentuated, and segregating the mul iply-reflected beam of light and comparing the same with an appropriate standard; substantially as described.

2. A method for accentuating tints which comprises juxtapositioning two beams of light derived from a common source, one of said beams having first undergone a plurality of reflections from surfaces of the tint to be accentuated, and the other of said beams being non-selectively derived from said source; substantially as described.

3. A method for the purposes herein set forth which comprises juxtapositioning two beams of light derived from a common source, one of said beams having first undergone a'plurality of reflections from surfaces 'of the color to be tested and the other of surface of the color to be tested, subjecting said beam of light .to a plurality of reflections from surfaces ofthe color to be tested, passing said multiply-reflected beam of light through the reflecting region of a photometer cube, passing a beam of light derived from said source without selective -reflection through" the transparent region of said photometer cube, and juxtapositioning the't'wo beams of light from thephotometer cube for thepurposes of comparison; substantially as described.

5. A' method for the purposes herein set forth which comprises directing a beam of light from an appropriate source upon a device having surfaces of the color to be tested, subjectingsaid beam of light to aplurality of reflections from said surfaces, passing said multiply-reflected beam of light through the reflecting region of a photometer cube,

- passing a reflected beam of light derived from said source without selective reflection through the transparent region of said photometer cube, juxtapositioning the two beams of light from the photometer cube for the purposes of comparison, and adjusting the intensity of said non-selective beam of light until the juxtaposed beams are appropriately matched; substantially as described.

6. An apparatus for accentuating tints I comprising a source of light, reflecting means including surfaces of the tint to be tested, means for directiiig a beam of light from said source upon said reflecting means so that the beam undergoes a plurality of reflections from said surfaces, and means for segregating the thus multiply-reflected beam of light for comparison with an appropriate standard.

7. An apparatus for accentuating tints comprising a source of light, reflecting means I including surfaces of the tint to be tested, means whereby a beam of light from said source directed upon said reflecting means comprising a source of light,reflecting means including a pair, of spaced members having their facing surfaces of the tint to be tested, means whereby a beam of light from said source directed upon said reflecting means undergoes a plurality of reflections from said tinted surfaces, and means for directing the multiply-reflected beam of light into an appropriate field of view.

9. An apparatus for accentuating tints comprising a source of light,.reflecting means including surfaces of the tint to be tested,

means whereby a beam of light from said.

source directed upon said reflecting means undergoes a plurality of'reflections from said surfaces, means for directing the multiplyreflected beam of light into an appropriate field of view, and means for directin into said field of view in juxtaposition wit said multiply-reflected beam a second beam of light derived from said source without selective reflection. a a

10. An apparatus for accentuating tints comprising a source of light, reflecting means including surfaces of the tint to be tested, means whereby a beam of light from said source directed upon said reflecting means undergoes a plurality of reflections from said surfaces, means for directing the multiplyreflected beam of light into an appropriate field of view, means for directing into said field of view in juxtaposition with said multiply-reflected beam a secbndbeam of light derived from said source without selective reflection, and means for adjusting the intensity of said non-selective beam of light.

11. An apparatus for accentuating tints comprising a source of light, reflecting means including surfaces of the tint to be tested, means whereby a beam of light from said source directed upon said reflecting meansundergoes a plurality of reflections from said surfaces, and means including a photometer cube for directing the'multipl'y-reflected beam of light into an appropriate field of-view. l

12. An apparatus for accentuating tints i comprising a source of light,reflecting means -1nclud1ng a pair of spaced members havin their facing surfaces of the tint to be teste I X means for adjusting the spaced relation between said members, means whereby a beam.

undergoes a plurality of reflections from said surfaces, means including a photometer cube for directing the multiply-reflected beam of light into an appropriate field of View, and means inchiding, said photometer cube for directinginto said field of View in juxtaposition with said multiply-reflected beam of light a beam of light derived from said source without selective reflection.

In testimony whereof I aliix my signature.

Al](ilTS'l- HERMAN PFUND.

was) 

